High-pressure discharge lamps of the type to which the present invention relates are described, for example, in British Pat. No. 2,126,415, and German Patent Disclosure Document No. 32 42 840. Such lamps are suitable for use at low power levels, that is, in the order of between 35 W to 150 W rated power, so that they are also suitable for interior room illumination.
The lifetime of such lamps is limited by the aggressive fill used within the lamp vessel, which results in corrosion of the electrods. This problem is particularly acute if the fill includes a substantial component of tin halides. Due to the high loading, the coiled portion of the electrodes tends to deform, thus changing the spacing of the electrode tips, which results in changes of light outout and lamp power.